KR20070032931A - Method and apparatus for measuring pixel drive current and recording medium - Google Patents

Abstract

The method for measuring the pixel driving current according to the present invention includes a first step of measuring an offset current flowing through a wiring line when a plurality of pixels are all set to a non-lighting state, and only predetermined pixels of the plurality of pixels may be turned on. A second step of measuring the pixel driving current of the predetermined pixels from the difference between the current flowing in the wiring and the offset current, and repeating the second step to continuously execute the pixel driving current of the predetermined number of pixels of the plurality of pixels. And a fourth step of resetting all of the plurality of pixels to a non-lighting state, and a fourth step of repeating the first to third steps to measure the pixel driving current of the display device.

Description

Method and apparatus for measuring pixel driving current and recording medium {AN APPARATUS AND METHOD FOR MEASURING TFT PIXEL DRIVING CURRENT}

1 is a schematic diagram of a measurement device described in an embodiment of the invention.

2 is an exemplary diagram of internal circuitry of a display device of an embodiment of the invention.

3 is an operational flowchart of the measuring apparatus of the embodiment of the present invention.

4 is a graph showing the number of measured pixels and the change in offset current and measured current.

5 is a graph illustrating the voltage-current characteristics of transistors in a pixel.

6 is an exemplary diagram of internal circuitry of a display device.

7 is a schematic diagram of a measurement device described in another embodiment of the present invention.

8 is an exemplary diagram of internal circuitry of a display of another embodiment of the present invention.

9 is an operational flowchart of the measuring apparatus of the embodiment of the present invention.

10 is a graph showing the number of measured pixels and the change in offset current and measured current.

Fig. 11 is a circuit diagram showing an embodiment in which the present invention is used for an active matrix using an EL element replacement load.

FIG. 12 is a circuit diagram showing a load 19 of FIG.

The present invention relates to a method and apparatus for measuring a pixel driving current, and more particularly, to a method and apparatus for measuring a pixel driving current of a display device having a structure in which pixel driving currents of a plurality of pixels are distributed and supplied from a common wiring. It is about.

In a display device using a self-emitting light-emitting element such as an EL element, the light emitting element is sealed in an active matrix substrate that controls the brightness of each light emitting element to produce a display panel. . Self-luminous LEDs generally emit light at a luminance corresponding to the current flowing through the device (pixel drive current). The active matrix substrate has a function of controlling emission luminance by controlling the pixel drive current of each pixel. Pixel drive current is often controlled by a control voltage using a FET. That is, as shown in FIG. 6, the light emitting element 66 is connected to the drain terminal of the transistor 64 and the current supplied to the light emitting element 66 is controlled by controlling the drain source current using the gate voltage. A holding capacitor 65 is generally disposed at the gate terminal to keep the gate voltage constant. In addition, in order to minimize the amount of wiring inside the substrate, the pixel driving current supplied to the source terminal is designed to be distributed and supplied from one wiring 62A supplying the driving current to each pixel.

The control circuit on the active matrix substrate is created by a relatively unstable layer-forming step, such as sputtering on a glass substrate and its equivalents, so that each pixel on the substrate has the desired function before shipping the finished display device. You need to test whether you have. One of the test items is the measurement of the pixel drive current. This measurement is performed by the following procedure. First, set the holding capacitor 65 of the pixel to be measured to the desired voltage. The holding capacitor 65 is connected to the gate terminal of the transistor 64 that controls the pixel current and is allowed to flow between the drain and the source a set voltage, that is, a current corresponding to the gate voltage. The flow of pixel drive current is then measured. By determining whether the measurement result is within a desired current range, it is possible to determine whether the transistor 64 that controls the pixel drive current of the measured pixel is operating correctly. It is also possible to determine whether the display device has a predetermined characteristic by performing this type of measurement and making a quality determination on all the pixels on the substrate.

Naturally, when measuring the pixel drive current, it is preferable that the pixel drive current of each pixel is measured separately. However, as described above, since the pixel drive current is configured to be distributed and supplied from a single wiring 62A supplying the drive current, it is impossible to measure the current of only a predetermined pixel. As a result, the pixel drive current of the measured pixel is generally known by measuring the current flowing through the wiring supplying the drive current when one or more pixels to be measured in the display device are lit and the other pixels are unlit. Can be.

However, in semiconductor integrated circuits such as active matrix substrates, it is difficult to completely separate the pixels from each other, so that a small amount of leakage current is generated. Since there is no way to zero the pixel drive current between the drain and the source, very little leakage current flows even when no measurement is taken. Therefore, even when all the pixels are in the non-lighting state, some current flows into the wiring for supplying the driving current for supplying the pixel driving current to the plurality of pixels. This flow is called the offset current.

A similar technique is disclosed in Japanese Patent No. 3628014, in order to eliminate the influence of such offset current in pixel drive current measurement, when the pixels to be measured are lit and the pixel drive current is found, the offset current is transferred to the wiring 62A. Subtract from the flowing current. Currently, a method of subtracting an offset current component is a method in which a measured value including an offset current is converted into a digital value and the offset current is subtracted by data processing. However, it is necessary to measure the current including the offset current component by this method. Therefore, it is necessary to widen the measuring range of the ammeter, in which it is difficult to obtain accurate measured values. Therefore, there are other methods of placing a constant current circuit that cancels offset current in parallel with an ammeter, using hardware to offset the offset current, and using only the ammeter to measure only the pixel drive current.

However, the leakage current described above is generated not only between the drain and the source, but also from the holding capacitor 65. The leakage current from the holding capacitor 65 changes the voltage between the terminals of the holding capacitor. As a result, the gate voltage changes, and a current flows between the drain and the source according to the gate voltage. In other words, the current between the drain and the source is not only the leakage current due to the above-described insulation characteristics between the drain and the source, but the current is also generated by the change in the gate voltage generated by the leakage current from the holding capacitor 65. do. Of course, the leakage current due to the insulating characteristics is constant, but since the charging amount of the holding capacitor 65 increases with time while the pixel driving voltage is applied to the wiring 62A, the current generated by the change in the gate voltage. Increases with time while the pixel drive voltage is applied. However, the p-type transistor 64 has a voltage-current characteristic as shown in FIG. Therefore, when the value of the gate source voltage Vgs moves to the left from the intersection point of the coordinate axis, the absolute value of the drain source current Ids increases nonlinearly. Therefore, the offset current flowing to the wiring 62A for supplying the driving current rapidly increases with time. 5 shows an example of voltage-current characteristics of a p-type MOS transistor. The direction of current and voltage polarity changes depending on the polarity of the transistor.

There are, for example, 500,000 or more pixels in a display device (786,432 pixels in XGA), and therefore it takes time to measure the pixel drive current of the entire display device. Therefore, when the offset current generated by the change in the gate voltage is left, the offset current increases and an offset current exceeding the measured amount flows into the wiring 62A for supplying the drive current. When the measurement is performed in such a state that the value of the offset current is large, the measurement must be performed within a wide measurement range, and thus it is difficult to make a measurement with high accuracy. In addition, accurate measurement is impossible without the function of accurately canceling offset current that changes over time while the pixel drive voltage is applied to the wiring 62A. Therefore, there is a need for a measuring method and apparatus capable of removing the influence of offset current generated by a change in gate voltage and capable of performing accurate measurement of pixel drive current.

In the example of the prior art disclosed in Japanese Patent No. 3628014, the constant current circuit canceling the offset current is arranged in parallel with the ammeter, the offset current is canceled using hardware, and only the pixel drive current is measured using the ammeter. The dynamic range of the current drawn is narrowed and accurate measurements are made. Nevertheless, when a plurality of pixels are measured continuously, the rate of increase of the offset current value 41 increases nonlinearly with time as shown in FIG. 10. That is, the absolute value of the offset currents B1, B2, B3, B4 gradually increases, as in (B1) to (B2), (B2) to (B3), and (B3) to (B4), The rate of change also increases gradually. As a result, the dynamic range required for the constant current source to offset the offset current also increases, making it difficult to supply the current accurately. It is also possible that the offset current value of the pixel under test is outside of the specified value for a variety of reasons. As a result, as shown in Fig. 10A, the dynamic range of the drive current 42 of the pixel to be measured in the test is substantially constant, even though the high accuracy of the ammeter can be maintained, the accuracy of offset current cancellation. Is likely to decrease and decrease the measurement accuracy of the entire system.

The above-described problems include the first step of measuring the offset current flowing through the wiring when the plurality of pixels are all set to the non-lighting state, and the current and the offset current flowing through the wiring when only predetermined pixels among the plurality of pixels are turned on. A second step of measuring a pixel driving current of predetermined pixels from the difference between the second step and repeating the second step, continuously measuring the pixel driving current of a predetermined number of pixels of the plurality of pixels, and then The pixel driving current measuring method according to the present invention includes a third step of resetting all of them to a non-lighting state and a fourth step of repeating the first to third steps to measure the pixel driving current of the display device. Can be.

By the present invention, it is possible to provide a measuring method and apparatus which can eliminate the influence of the offset current generated by the control voltage and can improve the precision in the measurement of the pixel driving current.

An exemplary embodiment of the present invention will be described with reference to the drawings.

1 is a schematic diagram of a pixel drive current measuring device 20 according to the present invention. The measuring device 20 includes a pixel control device 22 for controlling the lighting state of the pixels of the EL display device 10, which is a self-luminous display element, and a pixel driving voltage to a wiring for supplying a drive current to the display device 10. A power source 24 for applying the power source, an ammeter 23 disposed between the wiring for supplying the drive current and the power source 24, and the measurement control device 21 for controlling the operation of the measurement device 20. Include.

The pixel control device 22 has a function of specifying a pixel to be measured in the display device 10, controlling the lit / non-illuminated state of the measured pixel, and controlling the emission luminance of the pixel to be measured. In addition, the measurement control device 21 has an MPU 21A as a data processing means and a hard disk memory 21B. A program in which the measurement control method of the present invention is recorded is stored in the memory 21B.

The display device to be measured is not limited to the EL display device 10, but an active matrix substrate having a function in which a pixel driving current is controlled by a control voltage, and a light emitting element having a property whose luminance is controlled by a driving current flowing through the element. Any display driven using is also possible. In addition, the data processing means of the measurement control device 21 need not necessarily be an MPU, and any device having a mathematical operation function of digital data, such as a DSP, is possible. The memory means does not necessarily have to be a hard disk, but any device capable of storing digital data, such as flash memory or RAM, is possible.

The structure of the EL display device 10 to be measured is shown in FIG. The EL display device 10 includes pixels 11 arranged in a matrix form, wiring 12A for supplying pixel driving currents, common line 12B of the holding capacitor 15, data line 12C, and pixel driving. A common line 12D for the current and a gate line 12E connected to each pixel. In this design the ammeter 23 and the power source 24 of the measuring device 20 are connected to the wiring 12A. The common line 12D for the pixel drive current is set to the same potential as the ground potential of the display device 20. Unless stated otherwise, in the description below, the voltage is the potential difference from the voltage of the common line 12D.

The pixel 11 includes a transistor 13 for selecting a measured pixel to be controlled, a transistor 14 for controlling the pixel driving current, and a holding capacitor for maintaining the gate voltage of the transistor for controlling the pixel driving current ( 15 and an EL element 16. The gate terminal of the transistor 13 for selecting a pixel is connected to the gate line 12E, the source terminal is connected to the data line 12C, and the drain terminal controls one end of the pixel driving current and the holding capacitor 15. Is connected to the gate terminal of the transistor 14. The transistor 14 for controlling the pixel driving current is connected to one end of the drain terminal and the holding capacitor 15 of the transistor 13 for selecting the pixel, the source terminal is connected to the wiring 12A, and the drain terminal is connected to the EL. It is connected to one end of the element 16.

One end of the holding capacitor 15 is connected to the drain terminal of the transistor 13 for selecting the pixel and the gate terminal of the transistor 14 for controlling the pixel driving current, and the other end is connected to the common line 12B. . One end of the EL element 16 is connected to the drain terminal of the transistor 14 that controls the pixel driving current, and the other end is connected to the common line 12D. The transistor 13 for selecting a pixel and the transistor 14 for controlling a pixel driving current are both p-type MOS transistors, but they may be n-type MOS transistors or transistors having a structure other than MOS.

The operation of the pixel 11 will now be described. The term " conductive state " in the present specification and claims means a low impedance state between the drain and the source of the transistor. In this embodiment, the transistor 13 for selecting the pixel and the transistor 14 for controlling the pixel driving current both have voltage-current characteristics as shown in Fig. 5, and thus the gate-source voltage has a value of 0V or less. Both transistors are in a conductive state when the gate voltage is controlled. 5 shows the voltage-current characteristics of the gate-source voltage and the drain-source current in the conducting state. The EL element 16 is in the emitting state when the transistor 14 controlling the pixel driving current is in the conducting state.

In contrast, the "non-conductive state" means a state in which the impedance between the drain and the source of the transistor is high. The pixel is in a nonconductive state when the gate-source voltage is above 0V. The EL element 16 is in a non-emitting state when the transistor 14 that controls the pixel driving current is in a nonconductive state. However, as shown above, even in the non-conductive state, the leakage current due to the insulating property flows unless the current between the drain and the source is reduced to zero.

Pixel 11 is selected by driving gate line 12E to 0V. Generally, a voltage of 10V is applied to the gate line 12E and only the gate line 12E selected by the pixel control device 22 is induced to 0V. As a result, the transistor 13 which selects the pixel is in the conducting state, and the control voltage of the data line 12C is applied to the holding capacitor 15. The control voltage (emission luminance signal) is then provided from the pixel control device 22 to the data line 12C. When the control voltage is 5 V or more, the EL element 16 is in an unlit state and when the control voltage is less than 5 V, the EL element is in a lit state. In the lit state, the luminance gradually increases as the control voltage decreases, and the device emits light at maximum intensity when the voltage is 0V. A voltage of 5V is always applied to the common line 12B.

The holding capacitor 15 holding the control voltage is connected to the gate terminal of the transistor 14 which controls the pixel driving current, so that a pixel driving current corresponding to the control voltage is connected between the drain and the source of the transistor 14. Flow. The pixel drive current is supplied from the wiring 12A through which the pixel drive voltage is applied to the EL element 16 via the transistor 14.

Next, the operation of the pixel drive current measuring device 20 will be described. 3 is a flowchart showing the operation of the measuring device 20. The measurement consists of two measurements, in the preliminary measurement, after the holding capacitors of all the pixels of the display panel 20 are set to non-lighting state in the memory 21B (step 30), the offset current and the number of measured pixels A table showing the correlation between is generated and then the actual measurement (steps 31 to 36) by the measuring method of the present invention is made.

As described above, the offset current changes during the time that the pixel driving voltage is applied to the wiring 12A after the holding capacitors of all the elements are set to the non-lighting state. Therefore, in essence, it measures the correlation between the time the voltage is applied and the offset current, the time the pixel driving voltage is applied during this measurement, and the pixel driving time and the holding capacitor of all the pixels are set to non-lighting state. It is necessary to find the offset current from the time between when the current is measured. However, by the device 20 for measuring the pixel drive current, the pixel drive current is measured at a constant timing and the pixel drive voltage is continuously applied during the measurement, and thus the time at which the pixel drive voltage is applied and the measured pixel The number is proportional. As a result, the number of pixels measured is used as a preliminary measure of surrogate versus time while the pixel drive voltage is applied. As a result, by the apparatus for measuring the pixel drive current at non-uniform measurement timing, as described above, it is necessary to find a correlation between the pixel drive voltage and the time when the offset current is applied.

In the preliminary measurement (step 30), the holding capacitor of all the pixels of the display panel 10 (i.e., the gate terminal of the transistor 14 controlling the pixel drive current) is set at 5V (non-illuminated state) and the wiring ( The drive current flowing to 12A) is measured by the ammeter 23. The measured current value is an offset current value when the number of measured pixels is zero. Next, the holding capacitor 15 of the appropriate pixel (i.e., the gate terminal of the transistor 14 controlling the pixel drive current) is set to 3V (lit) and then reset to 5V (non-lit), The drive current flowing through the wiring 12A is measured by the ammeter 23. The measured current is the offset current when the number of measured pixels is one. At this time, the control voltage is set at the same timing as the actual measurement starting at step 31.

The same lighting / non-lighting operation is performed on the pixels at the same timing as the actual measurement so that the offset current is measured, and the offset current when the number of measured pixels is two is known. The optimal position of the pixel measured by the preliminary measurement is chosen to be as same as the state of the actual measurement which will be described later, if possible, but is not limited to this position depending on the state. For example, when only the number of measured pixels is important, the pixel may be in a completely different position, including the same position as the pixel for a known offset current when the number of measured pixels is one. The on / off operation of the pixels is repeated and the correlation between the number of measured pixels and the offset current is recorded in a table in the memory 21B.

As described above, the voltage of the holding capacitor 15 of the other pixels on the display device 10 (the gate voltage of the transistor 14) changes with the leakage current as the on / off operation of the pixels is performed, and each The current between the drain and the source of the pixel increases. Therefore, the offset current when the number of measured pixels is one is larger than the offset current when the number of measured pixels is zero. In addition, the offset current changes abruptly as the number of pixels measured increases over time.

When the change in the offset value of the display device 10 is known in advance, the preliminary measurement is not necessary and the actual measurement can be performed after the table is stored in the memory 21B. Further, when the pixel drive current is found by actual measurement using the correlation between the time during which the pixel drive voltage is applied and the offset current, all the pixels are set to the non-lit state, and the pixel drive voltage is connected to the wiring 12A. The offset current is measured at each predetermined time interval, and then the correlation between the offset current of the table and the time during which the measurement is performed can be recorded.

Next, actual measurement, which is a measuring method according to the present invention, will be described (steps 31, 32, 34, 38, 35, 36). In the actual measurement, the holding capacitor of all the pixels of the display device 10 is 5V. It is set (step 31). During this step, there is no current flowing to the transistor 14 which controls the pixel drive current of all the pixels, except for the leakage current between the drain and the source. The holding capacitor 15 of (11) is set to 3V (step 32) The voltage set at this time can be set as needed according to the measurement state, but is set to 3V in the measurement state in this embodiment. .

In addition, the current flowing through the wiring 12A is measured by the ammeter 23 (step 34). Next, using the data when the number of measured pixels in the offset current value table stored in the memory 21B is zero, the offset current value is subtracted from the measured value so that the measured value of the pixel drive current is known (step 38). ).

The measured current is stored in the memory 21B along with the pixel position (one row, one column) and the gate voltage 3V. The holding capacitor 15 of the measured pixel 11 is subsequently set to 5V (non-illuminated state).

Next, the pixel drive current of the pixel 17 measured in two rows and one column is measured. First, the holding capacitor of the measured pixel 17 is set at 3V (step 32). The current flowing into the wiring 12A is then measured by the ammeter 23 (step 34). Next, using the offset current value data when the number of measured pixels in the offset current value table stored in the memory 21B is 1, the offset current value is subtracted from the value measured by the ammeter so that the pixel drive current value is known ( Step 38). Known measurements are stored in memory 21B along with the pixel position (two rows and one column) and the gate voltage (3V). At this time, the measured value stored in the memory 21B is the difference between the current value flowing through the wiring 12A and the offset current value. Finally, the holding capacitor 15 of the measured pixel 17 is set at 5V (non-illuminated state). The pixel drive currents of all the pixels in the first column are measured continuously by this same process.

When the measurement of the pixels in the first column is completed (step 35), the holding capacitors of all the pixels of the display device 10 are reset to 5V (non-lighting state (step 31)). By this resetting, the pixels return to a state in which there is no current flowing to the transistor 14 which controls the pixel driving current of all pixels except the leakage current between the drain and the source. The process of steps 32, 34, 38 is then repeated and the pixel drive current of each pixel in the second column is measured continuously. At this time, the values measured in step 38 are calculated by calling the offset current value corresponding to the number of measured pixels from the memory 21B. For example, when pixels in one row and two columns are measured, the offset current value is set to the value when the number of measured pixels is zero, and when pixels in two rows and two columns are measured, the offset current value is the number of measured pixels. It is set to the value when 1.

In this way, when each pixel is measured continuously so that all the pixels on the display device 10 have been measured (step 36), the measuring operation of the measuring device 20 is completed. The MPU 21A is used to confirm whether the measurement value of each pixel stored in the memory 21B is within a required standard range and to determine the quality of the display device 10.

When the pixel drive current is measured using the correlation between the time during which the pixel drive voltage is applied and the offset current, the time since the plurality of elements are all set to the non-lighting state is known and the measured values are stored in the memory 21B. This is corrected using the offset current value corresponding to the resulting time from the table stored in. If the offset current value corresponding to the elapsed time is not input to the table, the offset current value can be found by using the offset current corresponding to the most recent time or by inserting data using the MPU 21A.

4 shows the change in the offset current when the voltage of the holding capacitor 15 is reset to the non-lighting state during the measurement process according to the embodiment of the present invention (the solid line 40) versus the offset current when the measurement is continued without resetting. The change (dotted line 41) is shown. For convenience, the solid line 40 and the dotted line 41 are shown as straight lines and curves, but the actual offset current value is the physical quantity obtained by the preliminary measurement as described above, and is input into the table and is strictly discontinuous. admit. Further, although the dotted line 41 showing the measured drive current value is shown step by step, this is merely a schematic illustration of the measured value of the object to be tested, and the position of each point in the figure has no special meaning. As can be clearly seen from the figure, as a result of the reset, the offset current value is periodically returned to the initial value, so that the increase of the offset current is controlled during the measurement process, and the dynamic range of the offset current is the range shown by C in the figure. Can be maintained within. The dynamic range of the measured drive current is the range shown by A in the figure, and the dynamic range required for the ammeter 23 can be maintained within A + C, ie the range shown by D in the figure. Therefore, the accuracy of the measurement can be prevented from decreasing. In addition, one row is measured each time the offset current returns to the initial value. As a result, the table in which the change of the offset current is recorded during the measurement process becomes unnecessary, and the contents of the table can be reduced.

In the embodiment of the present invention, the time during which the voltage of the holding capacitor 15 is reset to the non-lighting state during the measurement process is the time before the measurement of one column of pixels of the EL display device 10 is completed and the measurement of the next column is started. to be. However, the reset time is not limited to this example. For example, the voltage may be reset at any point during the measurement of the first column, or may be reset after the measurement of the plurality of columns. In addition, the time for which the voltage is reset can be predetermined so as to remain within the measurement range of the ammeter 23. It is also possible to monitor the measured value of the ammeter 23 using the measurement control device 21 to reset the voltage when a predetermined value is exceeded.

This embodiment has been described for the case where the offset current value for each pixel is known by preliminary measurement, but when the device having a small change over time in the offset current is measured, when the number of measured pixels is zero (initial value) The pixel drive current can be obtained by finding the difference between the current flowing through the wiring 12A and the offset current. In this case, the preliminary measurement is simplified (only the offset current when the number of measured pixels is zero is measured), and fast measurement is possible. In addition, there is an advantage that a large table is not necessary and thus the storage capacity of the memory 21B can be further reduced.

Another embodiment of the present invention will be described below with reference to the drawings.

7 is a schematic diagram of a pixel driving current measuring device 80 according to the present invention. The measuring device 80 includes a pixel control device 82 for controlling the lighting state of the pixels of the EL display device 70, which is a self-luminous display element, and a pixel driving voltage to the wiring for supplying a drive current to the display device 70. An ammeter 83 disposed between the power source 84 for applying the power source, the wiring for supplying the driving current, and the power source 84, and a constant current circuit 85 connected in parallel with the ammeter 83. And a measurement control device 81 for controlling the operation of the measurement apparatus 80.

The pixel control device 82 has a function of specifying a pixel to be measured in the display device 70, controlling the lit / non-illuminated state of the measured pixel, and controlling the emission luminance of the pixel to be measured. In addition, the measurement control device 81 has an MPU 81A as a data processing means and a hard disk memory 81B. A program in which the measurement control method of the present invention is recorded is stored in the memory 81B. The constant current circuit 85 is a circuit having a function of allowing a constant current to flow, and may be a circuit (current source) for generating a predetermined current by itself, or the power source 84 (a current of the current flowing through the ammeter 83). May be a circuit (a circuit for controlling the current) that allows only a predetermined flow of current from the remaining portion).

The display device to be measured is not limited to the EL display device 70, but an active matrix substrate having a function in which the pixel driving current is controlled by a control voltage, and a light emitting element having a property whose luminance is controlled by a driving current flowing through the element. Any display driven using is also possible. In addition, the data processing means of the measurement control device 81 need not necessarily be an MPU, and any device having a mathematical operation function of digital data, such as a DSP, is possible. The memory means does not necessarily have to be a hard disk, but any device capable of storing digital data, such as flash memory or RAM, is possible.

The structure of the EL display device 70 to be measured is shown in FIG. The EL display device 70 includes pixels 71 arranged in a matrix, wiring 72A for supplying pixel driving currents, a common line 72B of a holding capacitor 75, a data line 72C, and a pixel. A common line 72D for the drive current and a gate line 72E connected to each pixel. In this design the ammeter 83 and power source 84 of the measuring device 80 are connected to the wiring 72A. The common line 72D for the pixel drive current is set to the same potential as the ground potential of the display device 80. Unless stated otherwise, in the description below, the voltage is the potential difference from the voltage of the common line 72D.

The pixel 71 includes a transistor 73 for selecting the measured pixel to be controlled, a transistor 74 for controlling the pixel driving current, and a holding capacitor for maintaining the gate voltage of the transistor for controlling the pixel driving current. 75 and an EL element 76. The gate terminal of the transistor 73 for selecting a pixel is connected to the gate line 72E, the source terminal is connected to the data line 72C, and the drain terminal controls one end of the pixel driving current and the holding capacitor 75. Is connected to the gate terminal of the transistor 74. The transistor 74 for controlling the pixel drive current is connected to one end of the holding capacitor 75 and the drain terminal of the transistor 73 for selecting the pixel, the source terminal is connected to the wiring 72A, and the drain terminal is connected to the EL. It is connected to one end of the element 76.

One end of the holding capacitor 75 is connected to the drain terminal of the transistor 73 for selecting the pixel and the gate terminal of the transistor 74 for controlling the pixel driving current, and the other end is connected to the common line 72B. . One end of the EL element 76 is connected to the drain terminal of the transistor 74 that controls the pixel driving current, and the other end is connected to the common line 72D. The transistor 73 for selecting a pixel and the transistor 74 for controlling a pixel driving current are both p-type MOS transistors, but they may be n-type MOS transistors or transistors having a structure other than MOS.

The operation of pixel 71 will now be described. The term " conductive state " in the present specification and claims means a state where the impedance between the drain and source of the transistor is low. In this embodiment, the transistor 73 for selecting the pixel and the transistor 74 for controlling the pixel driving current both have voltage-current characteristics as shown in Fig. 5, so that the gate-source voltage has a value of 0V or less. Both transistors are in a conductive state when the gate voltage is controlled. 5 shows the voltage-current characteristics of the gate-source voltage and the drain-source current in the conducting state. The EL element 76 is in the emitting state when the transistor 74 that controls the pixel driving current is in the conducting state.

In contrast, the "non-conductive state" means a state in which the impedance between the drain and the source of the transistor is high. The pixel is in a nonconductive state when the gate-source voltage is above 0V. The EL element 76 is in a non-emitting state when the transistor 74 that controls the pixel driving current is in a nonconductive state. However, as shown above, even in the non-conductive state, the leakage current due to the insulating property flows unless the current between the drain and the source is reduced to zero.

Pixel 71 is selected by driving gate line 72E to 0V. Generally, a voltage of 7V is applied to the gate line 72E and only the gate line 72E selected by the pixel control device 82 is induced at 0V. As a result, the transistor 73 that selects the pixel is in a conductive state, and the control voltage of the data line 72C is applied to the holding capacitor 75. The control voltage (emission luminance signal) is then provided from the pixel control device 22 to the data line 12C. When the control voltage is 5 V or more, the EL element 76 is in a non-lighting state and when the control voltage is less than 5 V, the EL element is in a lit state. In the lit state, the luminance gradually increases as the control voltage decreases, and the device emits light at maximum intensity when the voltage is 0V. A voltage of 5V is always applied to the common line 72B.

The holding capacitor 75 holding the control voltage is connected to the gate terminal of the transistor 74 which controls the pixel driving current, so that the pixel driving current corresponding to the control voltage is connected between the drain and the source of the transistor 74. Flow. The pixel drive current is supplied from the wiring 72A to which the pixel drive voltage is applied to the EL element 76 through the transistor 74.

Next, the operation of the pixel drive current measuring device 80 will be described. 9 is a flowchart showing the operation of the measuring device 80. The measurement consists of two measurements, in the preliminary measurement, after the holding capacitors of all the pixels of the display panel 70 are set to non-lighting state inside the memory 81B (step 90), the offset current and the number of measured pixels A table showing the correlation between is generated and the actual measurement (steps 91 to 96) by the measuring method of the present invention is made.

As described above, the offset current changes during the time that the pixel driving voltage is applied to the wiring 72A after the holding capacitors of all the elements are set to the non-lighting state. Therefore, in essence, it measures the correlation between the time the voltage is applied and the offset current, the time the pixel driving voltage is applied during this measurement, and the pixel driving time and the holding capacitor of all the pixels are set to non-lighting state. It is necessary to find the offset current from the time between when the current is measured. However, by the device 80 for measuring the pixel drive current, the pixel drive current is measured at a constant timing and the pixel drive voltage is continuously applied during the measurement, thus, the time at which the pixel drive voltage is applied and the measured pixel The number is proportional. As a result, the number of pixels measured is used as a preliminary measure of surrogate versus time while the pixel drive voltage is applied. As a result, by the apparatus for measuring the pixel drive current at non-uniform measurement timing, as described above, it is necessary to find a correlation between the pixel drive voltage and the time when the offset current is applied.

In the preliminary measurement (step 90), the holding capacitor of all the pixels of the display panel 70 (i.e., the gate terminal of the transistor 74 controlling the pixel drive current) is set at 5V (non-illuminated state) and the wiring ( The drive current flowing to 72A is measured by ammeter 83. The measured current value is an offset current value when the number of measured pixels is zero. Next, the holding capacitor 75 of the appropriate pixel (i.e., the gate terminal of the transistor 74 that controls the pixel drive current) is set to 3V (lit) and then reset to 5V (non-lit), The drive current flowing through the wiring 72A is measured by the ammeter 83. The measured current is the offset current when the number of measured pixels is one. At this time, the control voltage is set at the same timing as the actual measurement starting at step 91.

The same lighting / non-lighting operation is performed on the pixels at the same timing as the actual measurement so that the offset current is measured and the offset current when the number of measured pixels is two is known. In this case, the position of the measured pixel may be the same as the pixel for the known offset current value when the number of measured pixels is 1, or may be another pixel. The same lighting / non-lighting process of the pixels is performed similarly and the correlation between the number of measured pixels and the offset current is recorded in a table in the memory 81B.

As described above, the voltage of the holding capacitor 75 (gate voltage of the transistor 74) of the other pixels on the display device 70 changes with the leakage current as the lighting / non-lighting operation of the pixels is performed, The current between the drain and the source of the pixel increases. Therefore, the offset current when the number of measured pixels is one is larger than the offset current when the number of measured pixels is zero. In addition, the offset current changes abruptly as the number of pixels measured increases over time.

When the change in the offset value of the display device 70 is known in advance, the preliminary measurement is not necessary and the actual measurement can be performed after the table is stored in the memory 81B. Further, when the pixel drive current is found by actual measurement using the correlation between the time during which the pixel drive voltage is applied and the offset current, all the pixels are set to the non-lit state, and the pixel drive voltage is wired 72A. The offset current is measured at each predetermined time interval, and then the correlation between the offset current of the table and the time during which the measurement is performed can be recorded.

Next, the actual measurement, which is the measuring method according to the present invention, will be described (steps 91 to 96). In the actual measurement, the holding capacitor of all the pixels of the display device 70 is set to 5V (step 91). Besides the leakage current between the drain and the source during this step, no current flows to the transistor 74 which controls the pixel drive current of all pixels. Next, the holding capacitor 75 of the pixels 71 measured in one row and one column is set to 3V (step 92). The voltage set at this time may be set as needed according to the measurement state, but is set to 3V in the measurement state in this embodiment. Next, when the number of measured pixels is zero, the current of the constant current circuit 85 is set to the offset current value from the table stored in the memory 81B (step 93).

In addition, the current flowing through the wiring 72A is measured by the ammeter 83 (step 94). Therefore, among the currents flowing to the wiring 72A, the offset current flows through the constant current circuit 85 to the wiring 72A without passing through the ammeter 83. Only the pixel drive current of the measured pixel can be measured by the ammeter 83. As a result, it becomes possible to measure the pixel drive current more accurately within a smaller measurement range. The measured current is stored in the memory 81B along with the pixel position (one row, one column) and the gate voltage (3V). The holding capacitor 75 of the measured pixel 71 is subsequently set to 5V (non-illuminated state).

Next, the pixel drive current of the pixels 77 measured in two rows and one column is measured. First, the holding capacitor of the measured pixel 77 is set at 3V (step 92). Next, the current of the constant current circuit 85 is set to the offset current when the number of measured pixels obtained from the table stored in the memory 81B is 1 (step 93). The current flowing into the wiring 72A is then measured by the ammeter 83 (step 94). The measured current is stored in the memory 81B along with the pixel position (two rows and one column) and the gate voltage (3V). The holding capacitor 75 of the measured pixel 77 is subsequently set to 5V (non-illuminated state). The pixel drive currents of all the pixels in the first column are measured continuously by the same process.

When the measurement of the pixels in the first column is completed (step 95), the holding capacitors of all the pixels of the display device 70 are reset to 5V (non-lighting state (step 91)). By this resetting, the pixels return to a state in which no current other than the leakage current between the drain and the source flowing to the transistor 74 which controls the pixel driving current of all the pixels flows. The process of steps 92, 93, and 94 is then repeated and the pixel drive current of each pixel in the second column is measured continuously. After resetting of the memory 81B, the current of the constant current circuit 85 set in step 93 is set by calling an offset current corresponding to the number of pixels measured at this time. For example, when the pixels in one row and two columns are measured, the current in the constant current circuit 85 is set to the offset current when the number of measured pixels is zero, and when the pixels in the two rows and two columns are measured, the constant current circuit 85 is measured. ) Is set to the offset current when the number of measured pixels is one.

In this way, when each pixel is measured continuously so that all the pixels on the display device 70 have been measured (step 96), the measuring operation of the measuring device 80 is completed. The MPU 81A is used to check whether the measurement value of each pixel stored in the memory 81B is within a required standard range and to determine the quality of the display device 70.

When setting the current of the constant current circuit 85 in step 93, when the pixel drive current is measured using the correlation between the time during which the pixel drive voltage is applied and the offset current, the plurality of elements are all in an unlit state. Since the time that has passed since is set to < RTI ID = 0.0 > is known, < / RTI > If the offset current value corresponding to the past time is not input to the table, the offset current value can be found by using the offset current corresponding to the most recent time or by inserting data using the MPU 81A.

4 shows the change in the offset current when the voltage of the holding capacitor 15 is reset to the non-lighting state during the measurement process according to the embodiment of the present invention (solid line 40) vs. the offset current when the measurement is continued without resetting The change (dotted line 41) is shown. As can be clearly seen from the figure, as a result of the reset, the offset current value is periodically returned to the initial value, so that the increase of the offset current is controlled during the measurement process, and the dynamic range of the offset current is the range shown by C in the figure. Can be maintained within. The measured current is canceled by the constant current circuit 85 set to the offset current value, so the dynamic range required for the ammeter 83 is kept within the range shown by A in the figure. Therefore, the accuracy of the measurement can be increased. The offset current returns to its initial value each time one column is measured. Therefore, the table of the memory 81B that determines the current of the constant current circuit 85 can be secured by the number of pixels in one column. As a result, the table in which the change of the offset current is recorded during the measurement process becomes unnecessary, and the contents of the table can be reduced.

In another embodiment, the pixel drive current can be known by offsetting the increase in offset current from the correlation between the number of measured pixels (or the time each pixel drive voltage is applied to the wiring 72A) and the offset current. However, when the resetting of the process (step 91) is frequently used, or when the device is measured to show only a small change in offset current for a long time, the pixel drive current is measured with the current flowing into the wiring 72A and the measured pixel. It can be found by finding the difference between the offset current (initial value) when the number of zeros is zero. In this case, the preliminary measurement is simplified (only the offset current when the number of measured pixels is zero is measured), and there is no need to set each time the current of the constant current circuit 85 is measured, thus enabling fast measurement. . In addition, there is an advantage that a large table is not necessary and thus the storage capacity of the memory 81B can be further reduced.

Although the technical concept of the present invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications are possible without departing from the spirit and scope of the claims. For example, although an FET was used as an element for controlling pixel drive current in an embodiment of the present invention, a display element using another current control element, such as an operational amplifier circuit, may be used in the present invention. Further, in the embodiment of the present invention, the holding capacitors 15 and 75 for holding the control voltage are used and the EL elements 16 and 76 are periodically initialized (resetting the voltages of the holding capacitors 15 and 75). Although reset to the non-lighting state, other means may be used to suppress the increase of the offset current by applying a constant voltage and initializing the state of this application means and to periodically reset the EL elements 16, 76 to the non-lighting state. It may be.

In addition, in the embodiment of the present invention, a cycle for resetting to a non-lighting state is not necessary for each row, and if the change of the offset current with time is large, it may be reset for every few pixels, or if the change If is small, reset may be performed for some pixel columns. Therefore, once the preliminary measurement is completed (step 30 and step 90), reference can be made to the amount of change in the offset current and the measurement control device 21, 81 can be used to determine how many pixels should be reset. In addition, in the embodiment of the present invention, it is not necessary to reset all the pixels of the display device. Once the pixels have returned to the non-lighting state, they can be reset using only the pixels measured a predetermined number of times. Also in this embodiment, the pixels to be measured need not be adjacent pixels that are measured continuously. Some all the pixels may be measured or the pixels may be measured arbitrarily.

Although the EL display device 10 using the active matrix substrate after the EL element is formed has been described in this embodiment, the present invention is described before the EL element is formed, for example, the circuit described in Japanese Patent 2004-294,457. A measurement load that replaces the device (alternative load) can also be applied to circuits disposed on open circuit electrodes on the matrix substrate. In this case, the term " lighted " herein means a state of current control that causes the EL element mounted on the substrate to light up. 11 shows a portion of a circuit of an active matrix substrate with a substitution load. This circuit has an electrode 18 disposed where the EL element is to be formed and has a load 19 connected between the electrode 18 and the wiring 12B. Capacitor 19A, diode 19B, transistor 19C and the like may be used for load 19 as shown in FIG. When transistor 19C is used, a new gate line controlling the load value is placed on the active matrix substrate. In this circuit of Fig. 11, the same reference numerals are used for the same structural parts as the embodiment shown in Fig. 2, and thus detailed description is omitted. The circuit using an alternative load for the EL element on the substrate before the EL element is molded can also be used in the embodiment of FIG.

The way in which the preliminary measurement is performed and the offset current values are stored in the table is desirable to improve the measurement speed. However, the present invention is not limited to this example, and the offset current and the pixel drive current may be repeatedly measured for each pixel when very high accuracy is required for the measurement. In this case, the offset current is first measured in the non-lit state, the pixel is turned on to measure the pixel drive current, the difference between the offset current and the measured current is stored as a result, and the pixel is returned to the non-lit state. Not only when the pixels in one column are measured, but also when the number of measured pixels exceeds any number, when the measurement time exceeds any measurement time, when the measured value exceeds any value, or Similarly it is possible that all pixels are reset to the non-lighting state at the proper timing.

By the present invention, it is possible to provide a measuring method and apparatus which can eliminate the influence of the offset current generated by the control voltage and can improve the precision in the measurement of the pixel driving current.

Claims (12)

In the measuring method for measuring the pixel drive current of a display device having a wiring for supplying a drive current to a plurality of pixels, A first step of measuring an offset current flowing to the wiring when the plurality of pixels are all set to a non-lighted state; A second step of measuring pixel driving currents of the predetermined pixels from a difference between the current flowing through the wiring and the offset current when only predetermined pixels of the plurality of pixels are turned on; Repeating the second step to continuously measure pixel driving currents of a predetermined number of pixels of the plurality of pixels, and then resetting all of the plurality of pixels to the non-lighting state; A fourth step of repeating the first to third steps to measure pixel drive current of the display device; How to measure pixel drive current. The method of claim 1, The pixels have a pixel drive current control element that controls the pixel drive current based on a control voltage, The third step is performed to reset the plurality of pixels to the non-lighting state by resetting the control voltage to a voltage when the pixel drive current control element becomes nonconductive. How to measure pixel drive current. The method of claim 1, The difference between the current flowing through the wiring and the offset current can be known by mathematical calculations from the digital data of the measured value of the current flowing through the wiring and the digital data of the offset current value of the target pixel. How to measure pixel drive current. The method of claim 1, The first step is a step of measuring a correlation between the time the pixel driving voltage is applied to the wiring and the offset current, The second step is to find the offset current from the correlation found in the first step by using the time the pixel driving voltage is applied to the wiring since all of the plurality of pixels are set to the non-lighting state. Containing How to measure pixel drive current. The method of claim 1, The first step is measuring the correlation between the number of measured pixels and the offset current, The second step includes measuring the offset current from the correlation found in the first step by using the measured number of pixels after the plurality of pixels are all set to the non-lighting state. How to measure pixel drive current. The method of claim 1, The pixels are the substitution load for the measurement How to measure pixel drive current. 10. A computer readable recording medium comprising executable computer program instructions for causing a processing system to execute a measurement method of measuring a pixel drive current of a display device having a wiring for supplying a drive current to a plurality of pixels when executed, the method comprising: The measuring method, A first step of measuring the offset current flowing in the wiring when the plurality of pixels are all set to the non-lighting state; A second step of measuring pixel driving currents of the predetermined pixels from a difference between the current flowing through the wiring and the offset current when only predetermined pixels of the plurality of pixels are turned on; Repeating the second step to continuously measure the pixel driving current of a predetermined number of pixels of the plurality of pixels, and then resetting all the plurality of pixels to the non-lighting state; Repeating the first to third steps and including a fourth step of measuring pixel drive current of the display device; Recording media. A measurement apparatus for measuring a pixel drive current of a display device having a wiring for supplying a drive current to a plurality of pixels having a pixel drive current control element for controlling the pixel drive current based on a control voltage, A power source for supplying the drive current to the wiring; An ammeter disposed between the power source and the wiring; A pixel control device for supplying a signal for controlling a lighting state of each pixel of the plurality of pixels; A measurement control device having data processing means and memory means, The measurement control device may include a) measuring the offset current flowing through the wiring after setting all of the plurality of pixels to a non-lighting state, and b) the current flowing through the wiring when each of the pixels is turned on, and A second step of continuously measuring a driving current of each of a predetermined number of pixels among the plurality of pixels based on the difference of offset currents; and c) repeating the first step and the second step, Used to implement a third step of measuring the pixel drive current of the pixels Pixel drive current measurement device. The method of claim 8, The pixel driving current control element is a transistor Pixel drive current measurement device. The method of claim 8, The measuring device further includes a constant current circuit connected in parallel with the ammeter and a constant current, such as the offset current, flows into the constant current circuit. Pixel drive current measurement device. The method of claim 8, Said first step comprises using said pixel control device to set said control voltage to a voltage when said pixel drive current control element is in an unlit state; Pixel drive current measurement device. The method of claim 8, The pixels are the alternative load for the measurement Pixel drive current measurement device.

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